Hitherto, gas-barrier laminates including a plastic substrate and an inorganic thin film made of aluminum oxide, magnesium oxide, silicon oxide, etc., and formed on the surface of the plastic substrate by a vapor-deposition method, a sputtering method, a CVD method or an ion-plating method, have been extensively used in various applications such as packaging of products which should be shielded against various gases such as water vapor and oxygen, and packaging of food, industrial products and drugs for preventing deterioration in quality thereof. These gas-barrier laminates have been recently used not only in these packaging applications, but also as a component of transparent conductive sheets which are used in liquid crystal displays, solar cells, electromagnetic shielding members, touch panels, EL substrates or color filters. Moreover, the above gas-barrier laminates have also been widely used as printed packaging materials because such an inorganic thin film generally has a good wettability and, therefore, a good printability.
In order to enhance a gas-barrier property of the inorganic thin film of the above gas-barrier laminates, there have been proposed methods of subjecting the inorganic thin film to various surface treatments. For example, there is known the method of absorbing water in a vapor-deposited silicon oxide film used as the inorganic thin film and then heat-treating the film to enhance a gas-barrier property thereof (e.g., refer to Japanese Patent Application Laid-Open (KOKAI) No. 2-299826).
However, the gas-barrier laminate obtained by the above method has such a problem that when the inorganic thin film is rubbed in subsequent laminating and printing processes, or when the resultant laminate is folded or bent in a subsequent bag-making process, the inorganic thin film tends to be damaged, resulting in deteriorated gas-barrier property. In particular, in the case where white ink containing a titanium oxide pigment is used for printing, the inorganic thin film tends to be more severely damaged and, therefore, considerably deteriorated in gas-barrier property. In particular, if the inorganic thin film is damaged by printing, there tends to arise such a problem that a gas-barrier property of the laminate is considerably deteriorated when subjected to subsequent hydrothermal treatment.
In order to prevent the inorganic thin film from being damaged upon the above various processes, there have been proposed the method of forming a transparent primer layer made of a resin having a glass transition point of not less than 40° C. on the surface of the inorganic thin film (e.g., refer to Japanese Patent Application Laid-Open (KOKAI) No. 5-269914), the method of forming an aqueous anchor coat layer on the surface of the inorganic thin film (e.g., refer to Japanese Patent Application Laid-Open (KOKAI) No. 5-309777), the method of forming a layer made of an aqueous resin or an aqueous emulsion containing silicon oxide on the surface of the inorganic thin film (e.g., refer to Japanese Patent Application Laid-Open (KOKAI) No. 5-9317), the method of forming a primer layer made of a polyester resin, a polyurethane resin or a nitrocellulose resin having a glass transition temperature of 60 to 80° C. and a molecular weight of 10000 to 20000 on the surface of the inorganic thin film (e.g., refer to Japanese Patent Application Laid-Open (KOKAI) No. 7-137192), the method of forming a coating layer made of an organic polymer and inorganic fine particles on the surface of the inorganic thin film (e.g., refer to Japanese Patent Application Laid-Open (KOKAI) No. 10-71663), and the method of forming a coating layer made of a resin such as urethane and/or vinyl chloride-vinyl acetate copolymers on the surface of the inorganic thin film (e.g., refer to Japanese Patent Application Laid-Open (KOKAI) No. 2001-270026).
However, when a printing ink is applied onto the surface of the resin coating layer to form a printed layer thereon, there tend to be various problems such as ink bleeding due to penetration of a solvent contained in the printing ink into the resin coating layer, and cissing of the printing ink due to poor wettability of the resin coating layer. Thus, the conventional laminates which are subjected to printing, in particular, gradation printing, tend to suffer from poor appearance. Further, since an adhesion strength between the plastic substrate and the inorganic thin film is low, the resultant laminates tend to be deteriorated in hand-cutting property.
In addition, when subjecting the gas-barrier laminate obtained by applying a transparent primer onto the surface of the inorganic thin film to retort treatment, the inorganic thin film tends to be damaged by stress caused upon retorting, resulting in deteriorated gas-barrier property of the laminate. Further, there has also been proposed the method of applying a coating material comprising a polyfunctional isocyanate compound and a polyester polyol onto the surface of the inorganic thin film. However, in this method, it may be difficult to attain a sufficiently stable oxygen-barrier property after retorting.